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Tapes, Terms & Formats

A Basic Guide to Audio Recording

Not only do 3M produce a useful booklet explaining tape and recording so that the veriest dolt could twig it, they give it away free! Then they wonder if we'd like to serialise it... where's the catch?


Leading tape manufacturers 3M produce a useful guide to recording and tape, written and compiled by the 3M Professional Audio/Video Technical Service Group. This month we start a two-part serialisation of the guide, covering tape and recording history, analogue and digital formats and including an exhaustive reference glossary of technical and recording industry terms.

Audio recording formats have progressed and multiplied over recent years, particularly with regard to digital systems. This guide outlines the major formats currently in use, offers information as to their applications, and includes a glossary of terms associated with the recording end of the signal chain. 3M has been at the forefront of many developments in recording, including the introduction of the first digital multitrack system in the late 1970s, and the company's Technical Service Group has compiled Tapes, Terms and Formats as an accessible guide to audio. We hope you will find it useful.

Magnetic History



Magnetic material has been used to record audible information for almost 100 years. The technology has progressed steadily from the original steel wire recorders, through machines using steel ribbon, to the use of iron particles applied to a backing material. The original paper backing tape gave way to plastic film during the 1940s. Acetate was a commonly used backing material for the early tapes but, due to its tendency to stretch, it was eventually replaced for most applications by polyester film, which is still in use today. Further evolution of the backing film has resulted in coatings to reduce static build-up and to improve fast wind performance.

A tape's magnetic coating is its most critical component, and is affixed to the backing by means of an adhesive binder, the composition of which contributes significantly to a tape's long-term performance and reliability. A successful tape owes its reputation to the carefully researched blending of magnetic layer, binder, backing material and back-coating. The magnetic layer itself has also undergone considerable development, with the objective being to find a formulation which could record the broadest range of frequencies but with the minimum of intrusion from background noise.

Tape noise is an inherent aspect of analogue recording, and extensive research has been put into maximising achievable signal-to-noise ratios. Most of the earliest tapes used a simple iron oxide material but this has evolved and progressed over the years. The new digital recording formats require magnetic coatings capable of storing very high signal densities, and this has further influenced the development of new tapes.

Analogue Recording



At the heart of every tape recorder is an electromagnet called a magnetic head. The head is actually a coil of fine wire wound on an iron core whose poles are brought closely together to form an extremely narrow gap.

Some recorders use a single head for both recording and playback, with a second 'erase' head positioned ahead of the first. Many recorders have several heads, each performing a specific function. Ideally, individual heads are assigned to the erase, record and playback functions. Often the term 'head' is used to describe the pole surfaces. These are highly polished areas of the head that come into intimate contact with the oxide surface of the tape. Great care should be taken not to scratch the head surfaces.

Figure 1: Basic Audio Recorder.


The recording process begins at the microphone, where sound waves are picked up and translated into analogous currents of electrical energy. As the sound waves vary continuously, so do the electrical currents, hence the term analogue recording. After appropriate amplification, this energy is fed to the magnetic head of the tape recorder. As the tape moves past the pole pieces, the varying intensities of current fed to the head coil induce magnetic fields that build and decay in the intimate contact with the oxide surface. The oxide particles of the tape become magnetised by fields that are synchronous with the original sound waves. The tape, as it glides past the head at a very uniform speed, carries away permanent images of the magnetic activity of the head.

A recorded tape actually carries magnetic 'photographs' of field variation taking place at the record head. Such a stream of 'photographs', however, is affected by what might be called a 'magnetic astigmatism' — non-linearities in the way that a metallic medium lends itself to magnetic impression. This characteristic naturally causes severe distortion in portions of the audible frequencies of the recorded information and is corrected by applying a signal known as bias. This is a very high frequency (inaudible) signal, usually about 100kHz, that is applied at the record head along with the audible signals to be recorded. The bias signal, or 'bias' for short, performs two important functions during the record function. It (1) reduces distortion, and (2) maximises the amount of the audio signal recorded, increasing the signal-to-noise ratio.

Figure 2: Tape Magnetisation.


Because proper biasing is necessary to achieve optimum performance, some attention must be given to matching tape and bias. Although every tape type has an optimum bias, an exact biasing level is rarely a precise point. It is normally a compromise point at which the best possible trade-off of tape characteristics is achieved.

In playback, the tape is moved at the same speed past a magnetic head. In this mode the head is not driven by the amplifier but is used in a passive manner. As the recorded tape comes into contact with the playback head, the varying intensities of its magnetic fields induce voltages in the head that are analogous to the original recorded sounds.

During the record and replay process, however, signal losses are introduced by a number of mechanical and physical limitations, including head-to-tape separation, head geometry design and tape thickness. These signal losses can be compensated for during recording and corrected during replay by amplifying or attenuating signals of different frequencies, as appropriate. This process is known as equalisation. The equalisation characteristics required are defined by industry standards in order to facilitate good interchange of tapes on different recorders.

After equalisation, the replayed electrical signals are then amplified by the playback amplifier and fed to the loudspeaker — a transducer that transforms this electrical energy back into sound waves.

Digital Recording



Digital recorders are designed to overcome limitations of analogue recording techniques (distortion, noise, print-through, signal loss during dubbing, etc.). Digital recording is based on the conversion of the analogue signals into a stream of binary-coded pulses (1's and 0's).

The analogue signals from the source are measured, or sampled, at a very high rate, typically between 40,000 and 50,000 times per second, and then converted into a series of 1's and 0's. This process is called quantisation. The accuracy of the digital representation is related to the sampling rate and the quantisation level. The process of quantisation produces blocks of 1's and 0's known as words. Each character in a word is known as a bit (binary digit), and the number of bits that make up a word have a bearing on the accuracy of reproduction. Each bit corresponds to about 6dB of signal-to-noise ratio, so a 16-bit digital audio system can provide a signal-to-noise ratio of greater than 90dB. Digital information is recorded and stored on the tape as a series of pulses. The replay process is the reverse of recording, with the digital pulses being read from the tape and converted back into analogue signals.

Figure 3: Basic Digital Recorder.


On replay, some corruption of the digital data can occur due to tape dropout, debris, etc. The powerful systems used to code the digital information can detect these digital errors, and provide correction and compensation to the final audible signal.

Why Digital Or Analogue?



Advantages of using digital recorders include:

Copying (Dubbing): Once an audio signal is converted to a digital signal it can be copied many times without adding distortion. The reason for this is that effectively only 1's and 0's are being copied and errors in the signal can be corrected electronically. Each copy in analogue recording will add a little distortion and noise, which degrades the quality of the final result.

Wow and Flutter: In an analogue recorder, any variation in the speed of the tape during record and playback causes the pitch of the audio signal to change, and is often a very audible type of distortion. This does not occur in a digital recorder, because the audio signal from the tape is temporarily stored in a buffer (electronic memory). Each digital word is then taken out of memory at the same regular interval it was recorded at during the original sampling process.

Print-Through: Print-through is the transfer of a signal from one layer of tape to another in an analogue recording, which can cause an audible echo of that signal. This does not occur in digital recording, so print-through is not a problem.

There are, however, other considerations which can affect the choice of 'digital or analogue'. Many conflicting personal opinions exist about the 'sound' of digital and analogue recorders. Although on paper, digital systems would appear to offer a universal solution, some describe the overall sound of digital as 'clinical' and prefer the supposed 'warmth' of analogue.

Current state-of-the-art noise reduction systems for professional analogue recorders can produce a signal-to-noise ratio to rival digital recorders, and the new high-output analogue tapes further contribute to the situation. Equipment purchase and hire costs are also important factors. Consequently, it is commonplace to find major music projects recorded on analogue or digital recorders, or even using a combination of both for different instruments or effects.

Next month, we continue with a guide to the various tape formats and their uses, digital formats and disk systems.

Contact Information


Professional Audio/Video Group
3M United Kingdom PLC, (Contact Details).

3M European Technical Service, (Contact Details).




GLOSSARY OF RECORDING TERMS


AES: Audio Engineering Society. International audio industry organisation.

ALIGNMENT (LINE-UP): The process of adjusting recorder heads and electronics to give optimum tape performance.

ANALOGUE: Representation of a signal as a continuously varying wave form (c.f. digital).

APRS: Association of Professional Recording Services. UK audio industry organisation.

APRS LABELS: A colour-coded system of tape labelling and identification, introduced by APRS.

AZIMUTH ALIGNMENT: The adjustment of a head to allow exact alignment of the head gap with the recorded magnetic signals on the tape. This alignment is critical for optimum high frequency performance.

BALANCE: The placement of sound sources in a studio in relation to the microphones.

BANDWIDTH: The range of frequencies that a system is capable of reproducing.

BIAS: A high-frequency (beyond audibility) alternating current fed into the record heads to reduce distortion due to the inherent nonlinearity of magnetic systems.

BIT (BINARY DIGIT): The smallest unit of digital information, represented as a 1 or a 0.

BIT ERROR RATE: The ratio of the number of bits lost to the total number of bits recorded (e.g. a bit error rate of 10-6 means one lost bit from one million recorded bits).

BOUNCE: Copying signals from a recorded track to a new empty track.

BRPG: British Record Producers' Guild (recently changed name to RePro). Producers' wing of APRS.

CCIR (Comite Consultatif International des Radiocommunications): International standards organisation.

CD: Compact Disc.

CD-R: Write-once recordable CD.

CHANNEL: A single, complete sound or signal path through a recording system.

CHROMIUM DIOXIDE: Refers to the type of oxide used in some audio cassettes (Type II). Permits higher levels at high frequencies than standard ferric oxide.

CLONE: An exact digital replica of a digital recording.

COERCIVITY: The magnetic force required to reduce the level of magnetisation of a tape from full saturation to zero. Useful for comparing relative bias and tape erasure requirements. Usually expressed in Oersteds (Oe).

COMPRESSION: Reduction of signal amplitude.

CONTROL TRACK: A tape track containing a recording used by a servo mechanism in the recorder, to control the tape speed. Analogous to the sprocket holes in cine film.

CRCC (Cyclic Redundancy Check Code): A data check word mathematically derived from the recorded data for error detection. 'CRC' errors are the smallest degree of signal error and are electronically corrected on playback.

CROSSTALK: The unintentional and undesirable mixing of signals from one channel (or track) with another.

DASH (Digital Audio Stationary Head): A professional open-reel digital multitrack recording format, introduced by Sony.

DAT (Digital Audio Tape): A digital audio recording format using a cassette containing metal particle tape.

DCC (Digital Compact Cassette): A digital cassette recording system, developed by Philips.

DECIBEL (dB): A unit of measurement used to express the loudness of sound. More generally, a unit for expressing the ratios of two powers, voltages, or currents on a logarithmic scale.

DE-EMPHASIS: An operation, complimentary to pre-emphasis, whereby signal frequencies emphasised before recordings are restored to their original relative amplitudes.

DIGITAL: Representation of a signal as a series of binary coded pulses.

DIGITAL ERROR: Loss or damage to digital information during replay.

DIN (Deutscher Industrie Normenausschus): German standards organisation.

DISTORTION: Unwanted changes to the source signal due to limitations or non-linearities in the recording process.

DMS (Digital Mastering System): The first professional digital multitrack recording system, introduced by 3M.

DROP-IN (Punch-in): The process of inserting a recording by replaying the tape up to a chosen point and then switching one or more tracks into record mode.

DROPOUT: A momentary loss in the playback signal caused by a change in the spacing between the tape and recorder heads. Can be due to debris or imperfections in the tape surface.

DYNAMIC RANGE: The difference between the maximum output level and the noise; i.e. the range of levels that may be achieved without producing audible distortion or noise.

EQUALISATION (EQ): i) The selective amplification or attenuation of certain frequency bands to compensate for amplitude variations in an audio system; e.g. in a listening room or performance venue, ii) The selective amplification or attenuation of frequencies within a recorder to achieve an industry standard frequency response (e.g. CCIR). iii) The selective amplification or attenuation of certain frequencies during mix-down, for creative effect.

ERASURE (Demagnetisation): The process by which a magnetic signal on a tape is removed by an external magnetic field.

FERRIC: Refers to the type of oxide used in most analogue open reel tapes, and some audio cassettes (Type I).

FLAT RESPONSE: A specific form of Frequency Response, defined in terms of dB variation from absolute flatness over a specified frequency range (e.g. +/-2dB from 30 to 16,000 Hz). It is an indication of a system's ability to reproduce all audible frequencies supplied to it, maintaining the original balance between low, middle and high frequencies.

FLUTTER: Changes in signal pitch and level caused by short, rapid variations in tape speed.

FLUX DENSITY: The total number of lines of magnetic induction per unit area.

FOLDBACK: The process of feeding microphone or off-tape signals back to the performers via headphones or loudspeakers.

FREQUENCY RESPONSE (tape): Sensitivity performance measured over a range of frequencies.

HEAD: In a tape recorder, an electromagnet across which the tape is drawn, and depending on its function, performs the following function: i) erases a previous recording (Erase Head); ii) converts an electrical input into a magnetic signal (Record Head); iii) converts a magnetic input into an electrical signal (Playback Head).

HEAD GAP: The separation between the pole pieces in a magnetic head, filled with nonmagnetic material.

HEADROOM: The difference between the maximum output level of the tape, and the (reference) operating level.

IEC (International Electrotechnical Commission): International standards organisation.

LEADER TAPE: Non-magnetic coloured plastic tape, spliced to the beginning of a recorded tape.

MD (Minidisc): A digital miniature disc recording system, developed by Sony.

MAGNETO-OPTICAL: A system of recording using both magnetic and optical technology.

MASTER TAPE: Tape containing edited or approved material, usually in two-track format.

MAXIMUM OUTPUT LEVEL (M.O.L.): The highest level that can be recorded on a magnetic tape without producing audible distortion (usually without exceeding 3% third harmonic distortion).

METAL PARTICULATE (MP) TAPE: The magnetic coating contains fine metal particles as opposed to oxide-type particles. This helps to give a higher signal output, and greater signal-to-noise ratio.

MIDI (Musical Instrument Digital Interface): A language which allows communication between compatible electronic musical instruments, processors, sequencers, recorders and mixing consoles.

MODULATION: An interference effect between two signals.

MTC: MIDI Time Code.

NAB (National Association of Radio and Television Broadcasters): USA standards organisation.

NAB REELS: Tape reels conforming to a standard defined by NAB (relating to hub size, reel diameters, etc....)

NOISE: Unwanted, random, low-level sound created by electronic equipment, heads, and the tape surface. Noise often occurs as hum and/or hiss and may be reduced by good machine and tape design.

NOISE FLOOR: The level of noise in a system, below which signals will not be heard.

NOISE REDUCTION: Electronic processing circuits to improve the signal-to-noise ratio of a recording system. These include the Dolby and dbx systems.

nWb/m (Nano Webers per metre): The unit of measurement of the level of magnetic flux density, and hence the level of signal recorded on tape.

OPERATING LEVEL (Reference Level): A level of recording and playback based upon a standard reference level.

OVERDUB: Subsequent recording onto individual tape tracks.

OVER-RECORD: i) Re-record; i.e. to record new signals over previously recorded areas of tape, ii) A signal level above the maximum output level, producing audible distortion.

OXIDE: The magnetic particles which, when coated on a polyester backing, provide the storage layer for the recorded signal.

PCM (Pulse Code Modulation): A type of binary encoding used in digital audio.

PD (Professional Digital, Pro-Digi): A professional open-reel digital multitrack recording format, introduced by Mitsubishi.

PQ: Digital sub-code data recorded on a CD master tape, including track timing and location information.

PRECISION REELS: Tape reels built to finer tolerances and offering stronger flanges for improved tape wind and protection.

PRE-EMPHASIS: A process whereby certain frequencies are increased in level before recording, usually for improving signal-to-noise ratio.

PRINT-THROUGH: Undesired low level transfer of magnetic fields from layer to layer of tape on the reel. In extreme cases this can be heard as a pre- or post-echo.

QUANTISATION: The resolution of the amplitude of an analogue signal into a digital value. The accuracy of this conversion depends on the total number of bits in the digital word. Hence a 16-bit digital audio recorder would reproduce greater accuracy than an 8-bit recorder.

RAMP-TIME: A measure of the speed and smoothness with which a recorder begins recording.

RECORD DRIVE: The electrical signal current that flows through the windings of the record head.

RECORDING FORMAT: A technical description of a recording system including the tape width and number of tracks and channels, e.g. Half inch, 16-track analogue.

RETENTIVITY: The ability of a magnetic tape to remain magnetised after the magnetising force is removed.

SAMPLING RATE: In digital recording, the number of times a signal is measured per second, and converted into a number representing the measured level.

SATURATION: The condition reached in magnetic tape where the output no longer increases with increased input.

SENSITIVITY (Tape output): Indicates the relative tape output for a given input (record) level. Sensitivity data plotted over a range of frequencies gives frequency response.

SESSION TAPE: Tape containing original recorded material, usually in multitrack format, including complete and incomplete takes.

SIGNAL-TO-NOISE RATIO: The ratio of the signal level to the measured noise level.

SIGNAL-TO-PRINT RATIO: A ratio of the recorded signal level to the measured print-through level.

SMPTE: Society of Motion Picture and Television Engineers. USA organisation responsible for tape recording standards and recommended practices.

SPARS: USA equivalent of APRS.

SPLICING: The physical joining of two sections of tape by means of splicing tape.

SUPPLY REEL: The reel from which the tape is wound when the recorder is in the play, record or fast forward mode.

TAILS OUT: Tape stored on the take-up reel (i.e. with the beginning of the recording at the hub end) in order to minimise the effect of print-through.

TAKE-UP REEL: The reel onto which the tape is wound when the recorder is in the play, record or fast forward mode.

TAPE BACKCOAT: A conductive layer of coating on the underside of the tape, used to help the mechanical running of the tape, and minimise the build-up of static charge (which can cause dropouts).

TAPE BACKING (or base): The flexible material, usually polyester, which is used as the carrier for the magnetic oxide of the tape.

TAPE COMPRESSION: Compression/distortion effect when recording high levels of signal on an analogue tape.

TAPE FORMULATION: A description of the manufacturing recipe for tape, including oxide type, polyester backing and backcoat.

TAPE REEL: The tape carrier, comprising: i) a central hub, on which the tape is wound; ii) upper and lower flanges, to help give a smooth, uniform wind quality and to offer protection to the tape edges.

TAPE WIND: Describes the smoothness and uniformity of the tape mass when wound and stored on the reel. Bad winds, resulting in strands of tape protruding above the tape pack can cause damage to the tape edge, affecting edge track information.

TEST TAPE: (Alignment tape/Calibration Tape) A pre-recorded tape whose levels are used for tape head and electronic adjustment, to give optimum tape performance.

TIME CODE: A signal recorded on one track for time and point location control purposes.

TRACKS: Regions of the tape scanned by the heads. The number and width of tracks is determined in the recording format used.

WOW: Changes in signal pitch and level caused by slow variations in tape speed.



Previous Article in this issue

Floored Genius?


Recording Musician - Copyright: SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.

 

Recording Musician - May 1993

Feature

Previous article in this issue:

> Floored Genius?

Next article in this issue:

> Alan Parsons & Stephen Court...


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